This invention relates to printing means and, more particularly, to thermal printing means suited to impart an indicia to a workpiece, e.g., a mail envelope, wherein the thermal printing means can suitably accommodate workpieces of varying surface texture.
Thermal transfer or printing of an image to a workpiece is a known technology. Generally, thermal printing utilizes a thermal print head consisting of a linear array of "ON-OFF" heating elements. Each element can be individually actuated in binary response to a generated bit input signal. Customarily, a control signal is generated by a control means, such as a programmable microcomputer, wherein a series of byte codes are transmitted to the thermal print head gating the individual heating elements to either an "ON" or "OFF" state in response to the control signal. A thermal tape coated on one side with thermally sensitive ink is passed between the thermal print head and a traversing workpiece. In response to the gating pattern of the print head elements, a series of dots and spaces are created on the workpiece. As the gate information is sequentially transmitted to the thermal head in synchronized relationship to traversing thermal tape and workpiece, an image is thereby imprinted to the workpiece.
Thermal printing offers a most important advantage over die cast image transfer techniques, in that images transferred by thermal printing have a superior resolution quality. However, thermal printing is sensitive to the workpiece image transfer surface area texture or roughness. The sensitivity is predicated on the limitation that the thermal head cannot be subjected to high compression loads. Therefore, the contact pressure between the workpiece thermal tape and thermal head must be maintained at a relatively low level. A workpiece having a rough surface texture has reduced surface contact with the thermal tape due to radical variation of cross-section surface contour as compared to a workpiece having a smooth surface contact area. As a result, a workpiece with a rough surface texture subjected to thermal image transfer receives an image lacking in resolution and contrast.
It would be advantageous if rough workpieces could be imaged by thermal transfer techniques in a manner preserving the superior imaging capabilities of thermal printing. In addition thermal printers are programmable. The programmable capability of thermal printing systems allows imaging flexibility which is not achievable with conventional die cast methods.